The present invention generally relates to apparatuses for depositing a mono-molecular layer, and more particularly to an apparatus for depositing a mono-molecular layer on a surface of a substrate by immersion of the substrate into a liquid having the mono-molecular layer at a liquid surface.
Generally, when producing a semiconductor element, processes are required for depositing (coating) various layers on a substrate. For example, a resist coater is used when depositing a resist layer on a wafer. The resist coater rotates the wafer and drops a predetermined quantity of the resist onto the rotating wafer so as to deposit the resist layer on the wafer by use of centrifugal force.
However, when the resist coater is used, there are problems in that pin holes are easily formed in the resist layer and it is difficult to deposit a uniform resist layer on the wafer. These problems become notable at the peripheral portion of the wafer especially when the diameter of the wafer is large. In addition, there is another problem in that the resist is not used efficiently in the resist coater. That is, the quantity of the resist which is actually used to form the resist layer on the wafer is only a few percent of the total quantity of the resist used, and the remaining quantity of the resist is wasted. Furthermore, the rotation of the wafer needs to be controlled depending on the viscosity of the resist, and the control of the wafer is complex.
Accordingly, as another method of depositing layers on the substrate, there is a method of forming a mono-molecular layer (or film) at a liquid surface and depositing the mono-molecular layer on the substrate by immersing the substrate into the liquid. Such a mono-molecular layer is often referred to as a Langmuir Blodgett layer (or film). Depending on the characteristic of the substrate surface, the mono-molecular layer is deposited on the substrate surface either on the first down pass or the first up pass of the substrate through the air/liquid interface.
As will be described later on in the present specification, the conventional apparatus for depositing a resist mono-molecular layer on the substrate comprises an elevator mechanism for raising and lowering the substrate with respect to purified water having the resist mono-molecular layer at the water surface. In the case where the resist mono-molecular layer is to be deposited on both substrate surfaces, the substrate is immersed into the water and then raised with the substrate surfaces approximately perpendicular to the water surface. On the other hand, in the case where the resist mono-molecular layer is to be deposited on only one substrate surface, the substrate is lowered to the water surface with the substrate surface approximately parallel to the water surface, without substantially immerising the substrate into the water.
The mono-molecular layer at the water surface is extremely sensitive to vibration (dynamic force) and will be damaged when subjected to the vibration. However, according to the conventional apparatus, the substrate itself vibrates since it is raised and lowered by the elevator mechanism, and such vibration of the substrate is inevitable. As a result, the vibtration of the substrate acts on the mono-molecular layer as the substrate makes contact with the water surface, and there is a problem in that the mono-molecular layer is easily disturbed and damaged. In addition, since the elevator mechanism is located above the mono-molecular layer, dust particles and the like generated from the elevator mechanism fall on the mono-molecular layer and easily disturb and damage the monom-molecular layer.
On the other hand, in the case where the substrate is lowered to the water surface with the substrate surface approximately parallel to the water surface, without substantially immersing the substrate into the water, it is difficult to control the lowering of the substrate so that the mono-molecular layer will not be disturbed and damaged by the vibration caused by the contact of the substrate and the mono-molecular layer at the water surface.
Furthermore, when forming the mono-molecular layer at the water surface, the conventional apparatus collects the molecules in one direction. But in order to prevent the molecules from being distributed unevenly, that is, to prevent holes and molecule overlap, the molecules must be collected in the one direction with extreme care. Otherwise, the mono-molecular layer at the water surface becomes non-uniform, and it is impossible to deposit a uniform mono-molecular layer on the substrate surface from such a non-uniform mono-molecular layer.
Therefore, the conventional apparatus suffers disadvantages in that the mono-molecular layer cannot be deposited on the substrate surface with a high reliability.